All mammalian cells require cholesterol as a structural component of their cell membranes and for non-sterol end products. Cholesterol is also required for steroid hormone synthesis. The very property, however, that makes cholesterol useful in the cell membranes, its insolubility in water, also makes it potentially lethal. When cholesterol accumulates in the wrong place, for example within the wall of an artery, it cannot be readily mobilized and its presence leads to the development of an atherosclerotic plaque. Elevated concentrations of serum cholesterol associated with low density lipoproteins have been demonstrated to be a major contributing factor in the development and progression of atherosclerosis.
In mammals, serum lipoprotein is composed of cholesterol together with cholesteryl esters, triglycerides, phospholipids and apoproteins. Serum or plasma lipoprotein is comprised of several fractions. The major fractions or classes of plasma lipoproteins are very low density lipoprotein (VLDL), low density lipoprotein (LDL), intermediate density lipoprotein (IDL), and high density lipoprotein (HDL). These classes differ from one another in size, density and in the relative proportions of triglycerides and cholesteryl esters in the core, and in the nature of the apoproteins on the surface.
In mammals, serum cholesterol is derived from exogenous dietary sources as well as through endogenous synthesis. Endogenous synthesis of cholesterol involves a complex set of enzyme-catalyzed reactions and regulatory mechanisms generally termed the mevalonate pathway. Cells face a complex problem in regulating mevalonate synthesis because cholesterol, the bulk end product of mevalonate metabolism, is derived from plasma low density lipoprotein which enters the cell by receptor-mediated endocytosis, as well as from synthesis within the cell. Each cell must balance these external and internal sources so as to sustain mevalonate synthesis while avoiding sterol over accumulation. This balance is achieved through feedback regulation of at least two sequential enzymes in mevalonate synthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG--CoA) synthase and HMG--CoA reductase and also of LDL receptors. In the absence of LDL, mammalian cells maintain high activities of the two enzymes, thereby synthesizing mevalonate for production of cholesterol as well as the non-sterol products. When LDL is present, from exogenous sources, HMG--CoA synthase and reductase activity is repressed and the cells produce smaller amounts of mevalonate for the non-sterol end products.
Abundant evidence indicates that treatment of hyperlipoproteinemia will diminish or prevent atherosclerotic complications. In addition to a diet that maintains a normal body weight and minimizes concentrations of lipids in plasma, therapeutic strategies include elimination of factors that exacerbate hyperlipoproteinemia and the administration of therapeutic agents that lower plasma concentrations of lipoproteins, either by diminishing the production of lipoproteins or by enhancing the efficiency of their removal from plasma.
The most promising class of drugs currently available for the treatment of hypercholesterolemia act by inhibiting HMG--CoA reductase, the rate limiting enzyme of endogenous cholesterol synthesis. Drugs of this class competitively inhibit the activity of the enzyme. Eventually, this lowers the endogenous synthesis of cholesterol and, by normal homeostatic mechanisms, plasma cholesterol is taken up by LDL receptors to restore the intracellular cholesterol balance.
Relative to other cells in the body, liver cells play a critical role in maintaining serum cholesterol homeostasis by both releasing precursors of LDL and through receptor mediated LDL uptake from the serum. In both man and animal models an inverse correlation appears to exist between liver LDL receptors and LDL-associated serum cholesterol levels. In general, higher hepatocyte receptor numbers result in lower LDL-associated serum cholesterol levels. Cholesterol released into hepatocytes can be stored as cholesterol esters, converted into bile acids and released into the bile duct, or enter into an oxycholesterol pool. It is this oxycholesterol pool that is believed to be involved in end product repression of both the genes of the LDL receptor and enzymes involved in the cholesterol synthetic pathway.
Transcription of the LDL receptor gene is known to be repressed when cells have an excess supply of cholesterol, probably in the form of oxycholesterol. A DNA sequence in the LDL receptor promoter region, known as the sterol response element, appears to confer this sterol end product repression. This element has been extensively studied (Brown, Goldstein and Russell, U.S. Pat. Nos. 4,745,060 and 4,935,363) and appears to consist of a 16 base pair sequence that occurs 5' of the LDL receptor coding region. The sterol response element can be inserted into genes that normally do not respond to cholesterol, conferring sterol end product repression on the chimeric gene. The exact mechanism of this repression is not understood. There is, however, abundant evidence that polar intermediates in cholesterol biosynthesis and naturally occurring as well as synthetic hydroxysterols repress genes containing the sterol response element.
It has been suggested that a hydroxycholesterol binding protein serves as a receptor. When the receptor is bound to an oxysterol it acts on the sterol response element to control transcription through a mechanism that is similar to the action of members of the steroid hormone receptor super gene family.
In populations where coronary heart disease is a major health problem, the incidence of the disease is markedly lower in women than in men. This is particularly true in younger age groups, such as men and women between 35 and 44 years of age.
Generally, plasma lipoprotein metabolism is influenced by the circulating concentrations of gonadal steroids. Changes in serum estrogen and androgen concentrations, resulting from alterations in gonadal status or from the administration of exogenous gonadal steroids are associated with changes in serum lipoprotein levels. The changes effected by estrogens and androgens generally support the proposition that sex differences in lipoproteins are due to hormonal differences between men and women.
The generally accepted relationship between gonadal steroids and plasma lipoproteins is that androgens lower HDL concentrations and increase LDL, thus contributing to the low HDL and high LDL levels observed in men when compared to women. Estrogens are held to have opposite effects on lipoproteins; that is, HDL is raised and LDL is lowered. These sex steroid-induced differences in lipoprotein concentrations are thought to contribute to the lower incidence of cardiovascular disease in women compared to men. After the menopause, the protective effect of estrogens in women is lost and the incidence of cardiovascular disease increases towards the male levels. Postmenopausal women who take estrogens generally have lower rates of cardiovascular disease than women of a similar age who do not. Estrogen, particularly when taken orally, lowers plasma levels of LDL and raises those of HDL.
The mechanisms by which estrogen lowers levels of LDL and raises those of HDL are not known. In general, changes in the plasma concentration of a lipoprotein result from changes in the rate of its synthesis or the rate of its catabolism. For example, estrogen may lower LDL levels by increasing the clearance of LDL from plasma, since estrogen increases the number of hepatic LDL receptors in animals.
Although estrogens have beneficial effects on serum LDL, given even at very low levels, long-term estrogen therapy has been implicated in a variety of disorders, including an increase in the risk of uterine cancer and possibly breast cancer, causing many women to avoid this treatment. Recently suggested therapeutic regimens, which seek to lessen the cancer risk, such as administering combinations of progestogen and estrogen, cause the patient to experience regular bleeding, which is unacceptable to most older women. Furthermore, combining progesterone with estrogen seems to blunt the serum cholesterol lowering effects of estrogen. Concerns over the significant undesirable effects associated with estrogen therapy, support the need to develop alternative therapies for hypercholesterolemia that generates the desirable effects on serum LDL but does not cause undesirable effects.
Attempts to fill this need by the use of compounds commonly known as antiestrogens, which interact with the estrogen receptor and/or bind what has been termed the antiestrogen binding site (AEBS), have had limited success, perhaps due to the fact that these compounds generally display a mixed agonist/antagonist effect. That is, although these compounds can antagonize estrogen interaction with the receptor, the compounds themselves may cause estrogenic responses in those tissues having estrogen receptors such as the uterus. Therefore, some antiestrogens, such as Tamoxifen, are subject to the same adverse effects associated with estrogen therapy.
The current invention provides methods for lowering serum LDL without the associated adverse effects of estrogen therapy, and thus serve as an effective and acceptable treatment for hypercholesterolemia.